Hundreds of angry protesters faced off with security police in the streets of Uganda's capital city of Kampala on Friday morning.
"At least four people were killed, 120 were injured and 360 were arrested during the riots," Kirunda Kivejinja,
Ugandan minister of internal affairs, said at a news conference.
Police used water canons to spray tear gas on the crowds while protesters set tires aflame in the heart of Kampala's financial district.
Rioters fueled by their growing discontent with the government and violent arrest of opposition leader Dr. Kizza Besigye chanted "Museveni must go!"
Besigye has been leading a "walk to work" campaign against rising food prices and an increased cost of living.
Ugandan President Yoweri Museveni warned that anyone taking part in the walk would be breaking the law and will be prosecuted.
"You dare not challenge the police, you dare not, you should humble yourself," warned Kivejinja in the televised press conference.
Besigye has been arrested numerous times this month. After his fourth arrest Wednesday, he was released on bail and ordered by the court not to protest for seven months. Defying court orders, Besigye hit the streets again Thursday, where men in civilian clothing confronted him, smashed his car and sprayed him with pepper spray before he was loaded in a police car.
Oppostion party officials told CNN on Friday that Besigye is en route to Nairobi for medical treatment.
"Clearly people have a right to demonstrate. Those demonstrations need to be lawful, but the arrest of Besigye yesterday was clearly incredibly violent, use of tear gas in a confined space, in his face the beating up of people nearby him, it's a serious situation and the police and the military services must bring an end to this kind of excessive use of force," said Maria Burnett, senior investigator for Human Rights Watch in Africa.
Besigye, a former doctor to Museveni, lost previous presidential bids in 2001 and 2006.
Besigye, the leader of the Forum for Democratic Change, lost a February election to Museveni. The incumbent has led the east African nation for 25 years.